THE U.S. POST OFFICE has never been one of my favorite charities. I do not, for example, write letters -- as my mother, who once sent a package of self-addressed stamped envelopes to me at college, will be the first to attest. I do use the postal service for paying the bills. But it recently took five days to deliver a check from one side of McLean to another, proving once and for all that the post office will do just about everything it is supposed to do except deliver the mail on time.

So when the idea surfaced this year to cut out Saturday mail sevice and thereby save $500 million, the reaction here was why not? Since we don't get The New Yorker, our Saturday mail was not much different from our Wednesday or Friday mail: a couple of pounds of junk mail and bills, none of which add a thing to our weekend.

For once, it seemed, Congress had come up with a way of cutting a lot of money out of the federal budget without taking food out of the mouths of old people and kids. Here was belt-tightening and budget-cutting that wouldn't hurt.

Saturday was the obvious day to cut because it is not a business day. Eighty percent of the mail delivered in the United States is business mail, according to David Minton, staff director of the House Post Office and Civil Service Committee. Cutting Saturday delivery would hurt the flow of business the least.

"The budget cutters said let's cut out a day. The only people who really care are the postal unions, because it means jobs, and small newspapers who rely on Saturday advertising for people to come to town and shop," said Minton. "They did not realize the deep reaction it would have. The unions are very influential with this committee. Rural America likes Saturday because they clip coupons and come to town and shop."

By now, Saturday, mail has become something of a cause celebre in the continuing saga of the federal budget. The mail has been coming in from constituents who vote and from newspaper owners who endorse. An effort to cut it out in the House Appropriations Committee failed last week, and Minton predicts it is not going to be eliminted "at this time."

Maybe not now, but the proposal has come up before and it will surely come up again. It raises important long-term questions about how we will be communicating 20 years from now and what will happen to those people who used to earn a living by carrying our communications across the country on pieces of paper. There are good arguments that go beyond questions of money in the budget to questions of opportunity in America.

There is, of course, the argument that eliminating Saturday service will cause all sorts of storage problems and will produce less reliable service. It won't reduce the volume of mail, it would only reduce the number of days for delivering it. For those of us who have given up on fast delivery of mail, this is not a particularly persuasive argument for keeping six-day service.

But there is something else. The postal service has traditionally been one of the most receptive government agencies to hiring blacks. It has been a way for people with only a high school edcuation to get a decent job, to get a piece of the action.

Postal jobs are jobs you can get on the basis of merit: you don't have to know somebody or be somebody's cousin. You take a test and if you pass, you can get a job. Along with blacks, Hispanics and Asians, a growing number of women are looking to postal jobs as a way up, according to George Gould, a lobbyist for the letter carriers. "It gives Americans who need some upward mobility a hell of a shot."

Gould, who's also former staff director of the post office committee, says the average carrier earns $18,000 a year for a five-day week. Should Congress cut Saturday mail on Oct. 1, which is one proposal, Gould says 20,000 carriers would be fired immediately, with an additional 20,000 postal jobs eliminated through attrition.

While Minton believes that closer to 15,000 jobs would be lost, he, too, points out that there is a serious social issue involved. "You're eliminating 15,000 to 20,000 jobs forever in the economy. Government postal jobs are good jobs. They won't come back at that level."

You can argue that if we're only keeping six-day postal service to provide jobs, then we ought to acknowledge that and call it a social service instead of a postal service. You can argue, too, that carrying messages around on pieces of paper will be obsolete in 20 years and that we are keeping a dying institution alive to the tune of $500 million a year. Soon, that argument goes, we will be keyboarding instead of stamping, but if that is true, we are really talking about hundreds of thousands of jobs.

Minton says that surveys have consistently shown that 75 to 80 percent of the people don't care about having Saturday mail. The popular mandate, despite the pressure from the newspaper owners and the postal unions, is to get rid of it and save some money. But it's not so simple in the short run and, if you believe in the electronic future, it's not so simple in the long run. When it comes to Saturday delivery, there is a lot more at stake than the mail.